Fixed bpo-29565: Corrected ctypes passing of large structs by value on Windows AMD64. (#168)

* Fixed bpo-29565: Corrected ctypes passing of large structs by value.

Added code and test to check that when a structure passed by value
is large enough to need to be passed by reference, a copy of the
original structure is passed. The callee updates the passed-in value,
and the test verifies that the caller's copy is unchanged. A similar
change was also added to the test added for bpo-20160 (that test was
passing, but the changes should guard against regressions).

* Reverted unintended whitespace changes.

2 files changed, 23 insertions, 0 deletions

diff --git a/Modules/_ctypes/_ctypes_test.c b/Modules/_ctypes/_ctypes_test.c
index 92b5adb..9410c7f 100644
--- a/Modules/_ctypes/_ctypes_test.c
+++ b/Modules/_ctypes/_ctypes_test.c
@@ -44,6 +44,19 @@ _testfunc_cbk_large_struct(Test in, void (*func)(Test))
     func(in);
 }
 
+/*
+ * See issue 29565. Update a structure passed by value;
+ * the caller should not see any change.
+ */
+
+EXPORT(void)
+_testfunc_large_struct_update_value(Test in)
+{
+    in.first = 0x0badf00d;
+    in.second = 0x0badf00d;
+    in.third = 0x0badf00d;
+}
+
 EXPORT(void)testfunc_array(int values[4])
 {
     printf("testfunc_array %d %d %d %d\n",
diff --git a/Modules/_ctypes/libffi_msvc/ffi.c b/Modules/_ctypes/libffi_msvc/ffi.c
index 1d82929..91a27dc 100644
--- a/Modules/_ctypes/libffi_msvc/ffi.c
+++ b/Modules/_ctypes/libffi_msvc/ffi.c
@@ -239,6 +239,16 @@ ffi_call(/*@dependent@*/ ffi_cif *cif,
       break;
 #else
     case FFI_SYSV:
+      /* If a single argument takes more than 8 bytes,
+         then a copy is passed by reference. */
+      for (unsigned i = 0; i < cif->nargs; i++) {
+          size_t z = cif->arg_types[i]->size;
+          if (z > 8) {
+              void *temp = alloca(z);
+              memcpy(temp, avalue[i], z);
+              avalue[i] = temp;
+          }
+      }
       /*@-usedef@*/
       return ffi_call_AMD64(ffi_prep_args, &ecif, cif->bytes,
 			   cif->flags, ecif.rvalue, fn);